Enhancing plasmonic hot-carrier generation by strong coupling of multiple resonant modes

Plasmon-induced hot carriers have recently attracted intensive research, but the energy efficiency in the visible light is often low due to the short lifetime of hot carriers and the limited optical absorption of plasmonic architectures. To boost the eneration of hot carriers, here we propose to exert multiple plasmonic resonant modes and their strong coupling using a metal-dielectric-metal (MDM) nanocavity that consists of an Au nanohole array (AuNHA), a TiO2 thin film and an Au reflector. Unlike common MDM structures, in addition to the Fabry-Pérot mode in the dielectric layer, the AuNHA as the top layer is special since it excites the localized surface plasmon resonance (LSPR) mode in the Au nanoholes and launches the gap surface plasmon polariton (GSPP) mode in the Au reflector surface. The spatial field overlapping of three resonance modes enables strong mode coupling by optimizing the TiO2 thickness, leading to notably enhanced average IPCE (~1.5%) and broadband photocurrent (170 μA∙cm^-2). This MDM structure would be useful for photochemistry and photovoltaics using sunlight.